1. Field of the Invention
The present invention relates to a protective layer transfer sheet. More particularly, the present invention relates to a protective layer transfer sheet which can provide a print, comprising a substrate having thereon an image, possessing excellent fastness properties.
2. Background Art
Halftone images and monotone images, such as letters and symbols, have hitherto been formed on a substrate by thermal transfer. Thermal transfer methods widely used in the art are thermal dye transfer and thermal ink transfer.
The thermal dye transfer is a method which comprises the steps of: providing a thermal transfer sheet comprising a substrate sheet bearing a dye layer formed of a sublimable dye as a colorant melted or dispersed in a binder resin; putting this thermal transfer sheet on the top of a substrate (optionally having a dye-receptive layer); applying energy corresponding to image information to a heating device, such as a thermal head, to transfer the sublimable dye contained in the dye layer onto the substrate, thereby forming an image.
For the thermal dye transfer, the amount of the dye to be transferred can be regulated dot by dot by regulating the quantity of energy applied to the thermal transfer sheet. Therefore, excellent halftone images can be obtained. In this method, however, unlike the formation of an image by a conventional printing ink using a pigment as the colorant, a relatively low-molecular weight dye is used as the colorant, and, in addition, a vehicle is absent. For this reason, the formed image is disadvantageously poor in fastness properties, such as light fastness, weather fastness, and rubbing fastness.
One method for solving the above problem of the prior art is to transfer a protective layer comprising an ultraviolet absorber or the like onto the formed image.
Some fastness properties of the image can be improved by this method. In the case of the conventional protective layer transfer sheet, however, the light fastness of the image is unsatisfactory. Cyan dyes are particularly likely to fade. Therefore, light irradiation leads to a lowering in density of the image and, at the same time, causes a change in hue to red, resulting in remarkably deteriorated image quality.
Another method for solving the above problem is to use an aromatic polycarbonate resin, capable of providing a print having an image, particularly a cyan dye image, possessing excellent light fastness, in a dye-receptive layer provided on a substrate (see, for example, in Japanese Patent Laid-Open Nos. 169694/1987 and 131758/1993). Further, improving the transferability of a dye onto a dye-receptive layer comprising an aromatic polycarbonate resin has also been disclosed (see, for example, in Japanese Patent Laid-Open Nos. 301487/1990 and 80291/1990).
Use of the aromatic polycarbonate resin as the protective layer in the protective layer transfer sheet is considered effective for solving the above problem. In this case, however, polycarbonate resins, derived from 2,2-bis(4-hydroxyphenyl)propane [bisphenol A] and represented by the following general formula, which have been described as preferred aromatic polycarbonate resins in most of the above publications, and copolymer polycarbonate resins disclosed in Japanese Patent Laid-Open No. 301487/1990 have low solubility in solvents, and chlorinated solvents, such as methylene chloride and trichloromethane, should be used in the production of the protective layer transfer sheet, posing a problem of work environment. ##STR1## wherein n is an integer.
Another problem involved in the conventional protective layer transfer sheet is that kick back is likely to be created. The kick back refers to such a phenomenon that, in the course of production of an integral transfer sheet, comprising protective layers and dye layers provided in a face serial manner on a common transfer sheet, involving a plurality of times of winding and rewinding, for example, the steps of rewinding the protective layer and the dye layer after coating, such as winding after the completion of coating, winding at the time of slittering after the coating, and winding around a bobbin as a form of a product, during storage in a wound state until next steps, the dye is first transferred (kicked) from the dye layer onto the backside of the substrate sheet, and, at the time of rewinding in the next step, the kicked dye is retransferred (backed) onto the front side of the substrate sheet facing the kicked dye. Rolls prepared in respective steps are different from one another in opposed faces. This creates a problem wherein each color dye is transferred onto the surface of the protective layer by the kick back phenomenon.
The creation of the kick back phenomenon in the transparent protective layer leads to a problem that transfer of the protective layer onto an image causes the image to be colored with the dye transferred by the kick back phenomenon, resulting in remarkably deteriorated image quality.
The present invention has been made under the above circumstances, and an object of the present invention is to provide a protective layer transfer sheet which can provide a print having enhanced light fastness properties.